Enhanced levels of vitamin B6 increase aerial organ size and positively affect stress tolerance in Arabidopsis

Vitamin B-6 is an essential nutrient in the human diet derived primarily from plant sources. While it is well established as a cofactor for numerous metabolic enzymes, more recently, vitamin B-6 has been implicated as a potent antioxidant. The de novo vitamin B-6 biosynthesis pathway in plants has recently been unraveled and involves only two proteins, PDX1 and PDX2. To provide more insight into the effect of the compound on plant development and its role as an antioxidant, we have overexpressed the PDX proteins in Arabidopsis, generating lines with considerably higher levels of the vitamin in comparison with other recent attempts to achieve this goal. Interestingly, it was possible to increase the level of only one of the two catalytically active PDX1 proteins at the protein level, providing insight into the mechanism of vitamin B-6 homeostasis in planta. Vitamin B-6 enhanced lines have considerably larger vegetative and floral organs and although delayed in prereproductive development, do not have an altered overall morphology. The vitamin was observed to accumulate in seeds and the enhancement of its levels was correlated with an increase in their size and weight. This phenotype is predominantly a consequence of embryo enlargement as reflected by larger cells. Furthermore, plants that overaccumulate the vitamin have an increased tolerance to oxidative stress providing in vivo evidence for the antioxidant functionality of vitamin B-6. In particular, the plants show an increased resistance to paraquat and photoinhibition, and they attenuate the cell death response observed in the conditional flu mutant.